1. Field of the Invention
The present invention relates to an active-matrix type liquid crystal display (LCD) for respectively driving pixels by thin film transistors(TFT) to perform a liquid crystal display operation, and more particularly to a reflection type liquid crystal display utilizing poly silicon formed through low-temperature process.
2. Description of the Prior Art
A liquid crystal display which is provided with a liquid crystal sealed between a pair of substrates and applies a voltage to the liquid crystal to perform desirable display operation has an advantage that it is small-sized and thin and can easily reduce power consumption. For this reason, currently, the liquid crystal display has been used practically as a display for various OA and AV device, portable or on-board information device, or the like. In particular, a so-called active-matrix type liquid crystal display using a thin film transistor (hereinafter referred to as a TFT) as a switching element for driving each liquid crystal pixel can selectively drive the TFT to select a liquid crystal pixel. Therefore, image display having no cross talk and a higher resolution can be obtained.
As the TFT to be used for the liquid crystal display, there have been known an amorphous silicon TFT using amorphous silicon for an active layer and a poly silicon TFT using poly silicon having a higher mobility for the active layer. The amorphous silicon TFT has mostly been used for a large-sized display and the like because it can be formed over a large area in a low-temperature process. On the other hand, the poly silicon TFT has a higher mobility than the amorphous silicon and can form an element in a self-aligned manner. Therefore, the poly silicon TFT can easily reduce a TFT area and a pixel area more than the amorphous silicon TFT so that a display having a high resolution can be manufactured. With the use of the poly silicon, the TFT can easily have a CMOS structure. For this reason, a driver TFT for driving a display section TFT can be formed on the same substrate in almost the same processes as the display section TFT.
As stated above, the poly silicon TFT has such attractive characteristics and can have a driver on its substrate. The poly silicon is formed by polycrystallizing amorphous silicon in a high-temperature process. The amorphous silicon is exposed to a high temperature during the process. Therefore, an inexpensive glass substrate cannot be used for the substrate. For this reason, it is difficult to use the poly silicon TFT in a practical situation.
However, a polycrystallization technology using an annealing treatment such as laser annealing, lamp annealing or the like has been enhanced. Consequently, it has been possible to manufacture the poly silicon in the low-temperature (below 600.degree. C.) process. In a method for forming the poly silicon TFT in the low-temperature process, thus, an inexpensive glass substrate can be used for a substrate. Therefore, cost can be reduced, and furthermore, an area can be increased. As a result, the poly silicon TFT (hereinafter referred to as a low-temperature poly silicon TFT) formed in the low-temperature process can be used practically.
As described above, the low-temperature poly silicon TFT has been used practically. However, optimum conditions for exhibiting the characteristics of the low-temperature poly silicon TFT to the utmost and for enhancing the characteristics still more with the use of a reflection type liquid crystal display for a projector or the like, for example, a liquid crystal material, an optimal panel structure and the like have not been developed. Accordingly, a material and a structure which have been used in a liquid crystal display for a conventional amorphous silicon TFT are utilized for other purposes. Consequently, the characteristics of the poly silicon TFT cannot be exhibited fully.